1. Field of the Invention
The invention relates to the field of metallurgy and particularly to the field of titanium base alloys having excellent superplastic forming properties.
2. Description of the Prior Art
Titanium alloys are widely used as aerospace materials, e.g., in aeroplanes and rockets since the alloys possess tough mechanical properties and are comparatively light.
Titanium alloys are difficult materials to work. When finished products have a complicated shape, the yield in terms of weight of the product relative to that of the original material is low, which causes a significant increase in the production cost.
To avoid these disadvantages, a new technology called superplastic forming which utilizes the superplastic phenomena, has been proposed. Superplasticity is the phenomena in which materials under certain conditions, are elongated up to from several hundred to one thousand percent, and in some cases, over one thousand percent, without necking down. Superplasticity can be classified in two categories, namely the transformation superplasticity which utilizes isothermal transformation of materials, and the fine-grain superplasticity which is found in materials in which grain size is very small. The industrially important superplasticity is the latter, that is, the fine-grain superplasticity.
One of the titanium alloys which possess superplastic properties is Ti-6Al-4V. U.S. Pat. No. 4,299,626 discloses titanium alloys in which Fe, Ni, and/or Co are added to Ti-6Al-4V to improve superplastic properties.
However, even with the Ti-6Al-4V alloy having a microstructure in which the grain size is in the fine range of 5 to 10 .mu.m, the temperature for superplastic forming is the high temperature of from 875.degree. to 950.degree. C., which shortens the life of working tools or necessitates costly tools. Even with the alloy described in U.S. Pat. No. 4,299,626, the said temperature can be lowered by only 50.degree. to 80.degree. C. compared with that for Ti-6Al-4V alloy, and the elongation obtained at such a temperature range is not sufficient.